spring对Annotation的派生性应用可谓炉火纯青，在spring core：@Component的派生性讲过支持层次上派生性，而属性上派生的需求则借助了@AliasFor，它是从spring4.2中开始支持的。
@AliasFor注解用于声明注解元素的别名，应用于方法上（别忘了注解本质是接口）。Spring框架在内部使用大量的使用这个注解，例如，@Bean，@ComponentScan，@Scope等。
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Documented
public @interface AliasFor {
/**
* * 引用的注解别名..
*/
@AliasFor("attribute")
String value() default "";
/**
* 引用的注解别名.
* @see #value
*/
@AliasFor("value")
String attribute() default "";
/**层次结构的父注解
*/
Class<? extends Annotation> annotation() default Annotation.class;
}示例1
可通过AnnotatedElementUtils#getMergedAnnotationAttributes来读取。下面演示没有派生性的情况：
@Test
public void metaTest() throws IOException {
AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home.class, AccessRole.class);
System.out.println(aa);//{value=super-user, module=gui, accessType=super-user}
}
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface AccessRole {
@AliasFor("accessType")
String value() default "visitor";
@AliasFor("value")
String accessType() default "visitor";
String module() default "gui";
}
@AccessRole("super-user")
//@AccessRole(value = "super-user",accessType = "super")//error
public class Home {
}注：alias references的默认值必须一致，使用时指定值也必须一致，否则会抛出AnnotationConfigurationException
下面演示有单层次及多层次派生性的情况：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@AccessRole("admin")
public @interface AdminAccess {
@AliasFor(annotation = AccessRole.class, attribute = "module")
String value() default "service";
}
@AdminAccess
public class Home2 {
}
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@AdminAccess("supper")
public @interface SupperAccess {
String value() default "service3";
@AliasFor(annotation = AccessRole.class, attribute = "module")
String module() default "service3";
}
@SupperAccess
public class Home3 {
}
@Test
public void metaTest() throws IOException {
AnnotationAttributes aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AdminAccess.class);
System.out.println(aa);//{value=service}
aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home2.class, AccessRole.class);
System.out.println(aa);//{value=admin, accessType=admin, module=service}
aa = AnnotatedElementUtils.getMergedAnnotationAttributes(Home3.class, AccessRole.class);
System.out.println(aa);//{value=admin, module=service3, accessType=admin}
}原理
AnnotatedElementUtils处理也不是特别复杂，很好理解
public abstract class AnnotatedElementUtils {
@Nullable
public static AnnotationAttributes getMergedAnnotationAttributes(
AnnotatedElement element, Class<? extends Annotation> annotationType) {
AnnotationAttributes attributes = searchWithGetSemantics(element, annotationType, null,
new MergedAnnotationAttributesProcessor());
AnnotationUtils.postProcessAnnotationAttributes(element, attributes, false, false);
return attributes;
}
//第一步：searchWithGetSemantics真正调用的方法，递归获取所有的注解
@Nullable
private static <T> T searchWithGetSemantics(AnnotatedElement element,
Set<Class<? extends Annotation>> annotationTypes, @Nullable String annotationName,
@Nullable Class<? extends Annotation> containerType, Processor<T> processor,
Set<AnnotatedElement> visited, int metaDepth) {
if (visited.add(element)) {
try {
// Start searching within locally declared annotations
List<Annotation> declaredAnnotations = Arrays.asList(AnnotationUtils.getDeclaredAnnotations(element));
T result = searchWithGetSemanticsInAnnotations(element, declaredAnnotations,
annotationTypes, annotationName, containerType, processor, visited, metaDepth);
if (result != null) {
return result;
}
if (element instanceof Class) { // otherwise getAnnotations doesn't return anything new
Class<?> superclass = ((Class<?>) element).getSuperclass();
if (superclass != null && superclass != Object.class) {
List<Annotation> inheritedAnnotations = new LinkedList<>();
for (Annotation annotation : element.getAnnotations()) {
if (!declaredAnnotations.contains(annotation)) {
inheritedAnnotations.add(annotation);
}
}
// Continue searching within inherited annotations
result = searchWithGetSemanticsInAnnotations(element, inheritedAnnotations,
annotationTypes, annotationName, containerType, processor, visited, metaDepth);
if (result != null) {
return result;
}
}
}
}
catch (Throwable ex) {
AnnotationUtils.handleIntrospectionFailure(element, ex);
}
}
return null;
}
}
public abstract class AnnotationUtils {
//第二步：获取@AliasFor的标记，处理其值
static void postProcessAnnotationAttributes(@Nullable Object annotatedElement,
@Nullable AnnotationAttributes attributes, boolean classValuesAsString, boolean nestedAnnotationsAsMap) {
if (attributes == null) {
return;
}
Class<? extends Annotation> annotationType = attributes.annotationType();
// Track which attribute values have already been replaced so that we can short
// circuit the search algorithms.
Set<String> valuesAlreadyReplaced = new HashSet<>();
if (!attributes.validated) {
// Validate @AliasFor configuration
Map<String, List<String>> aliasMap = getAttributeAliasMap(annotationType);
aliasMap.forEach((attributeName, aliasedAttributeNames) -> {
if (valuesAlreadyReplaced.contains(attributeName)) {
return;
}
Object value = attributes.get(attributeName);
boolean valuePresent = (value != null && !(value instanceof DefaultValueHolder));
for (String aliasedAttributeName : aliasedAttributeNames) {
if (valuesAlreadyReplaced.contains(aliasedAttributeName)) {
continue;
}
Object aliasedValue = attributes.get(aliasedAttributeName);
boolean aliasPresent = (aliasedValue != null && !(aliasedValue instanceof DefaultValueHolder));
// Something to validate or replace with an alias?
if (valuePresent || aliasPresent) {
if (valuePresent && aliasPresent) {
// Since annotation attributes can be arrays, we must use ObjectUtils.nullSafeEquals().
if (!ObjectUtils.nullSafeEquals(value, aliasedValue)) {
String elementAsString =
(annotatedElement != null ? annotatedElement.toString() : "unknown element");
throw new AnnotationConfigurationException(String.format(
"In AnnotationAttributes for annotation [%s] declared on %s, " +
"attribute '%s' and its alias '%s' are declared with values of [%s] and [%s], " +
"but only one is permitted.", attributes.displayName, elementAsString,
attributeName, aliasedAttributeName, ObjectUtils.nullSafeToString(value),
ObjectUtils.nullSafeToString(aliasedValue)));
}
}
else if (aliasPresent) {
// Replace value with aliasedValue
attributes.put(attributeName,
adaptValue(annotatedElement, aliasedValue, classValuesAsString, nestedAnnotationsAsMap));
valuesAlreadyReplaced.add(attributeName);
}
else {
// Replace aliasedValue with value
attributes.put(aliasedAttributeName,
adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
valuesAlreadyReplaced.add(aliasedAttributeName);
}
}
}
});
attributes.validated = true;
}
// Replace any remaining placeholders with actual default values
for (Map.Entry<String, Object> attributeEntry : attributes.entrySet()) {
String attributeName = attributeEntry.getKey();
if (valuesAlreadyReplaced.contains(attributeName)) {
continue;
}
Object value = attributeEntry.getValue();
if (value instanceof DefaultValueHolder) {
value = ((DefaultValueHolder) value).defaultValue;
attributes.put(attributeName,
adaptValue(annotatedElement, value, classValuesAsString, nestedAnnotationsAsMap));
}
}
}
}待续
